1. Field of the invention
The present invention relates to an optical disc data recording and reproducing apparatus which optically writes data onto or reads data from an optical disc. More precisely, the invention relates to a driving apparatus which electromagnetically drives an objective lens thereof.
2. Description of related Art
In a known optical disc apparatus, such as an opto-magnetic disc apparatus, an objective lens is held on a carriage which is movable in a radial direction (i.e., tracking direction) of the optical disc to be movable in the tracking direction and the focusing direction through an elastic member. To drive the objective lens in the tracking direction and the focusing direction, tracking and focusing coils are provided on a lens holder, which holds the objective lens, and a magnetic circuit which constitutes electromagnetic driving circuits together with the tracking and focusing coils is provided on the carriage. The focusing and tracking coils are supplied with predetermined directions and intensities of electric current to drive the objective lens holder so as to accurately converge the laser beams onto the recording track of the optical disc.
In such an electromagnetic objective lens driving apparatus, it is necessary to make the carriage and lens holder as light as possible to quickly access a designated track and to perform precise focusing and tracking operations. Therefore making the carriage and the lens holder small and light has been a theme in recent years. On the other hand, a thrust power of the electromagnetic driving apparatus is effected by an effective length of coils which is the thrust source, and a magnetic flux density which effects the coils. Thus, when a movable portion and the magnetic circuits are made small and light, characteristic fluctuation occurs in a driving operation of the objective lens by the electromagnetic driving circuits because of the decrease of linearity of the magnetic flux density which effects the electromagnetic drive specifically, the movable portion, especially a form of the objective lens holder is not stable, so that it is difficult to perform precise focusing and tracking operations. To solve this problem, it is necessary to increase the magnetic flux density so as to improve the electromagnetic driving circuit.
FIG. 16 shows an example of an electromagnetic driving circuit. The magnetic circuit is formed by a magnetic yoke 2 which is substantially U shaped, with a permanent magnet 1 which is provided on one end of the inner surface of the yoke 2 in which a line of magnetic induction is designated ".PHI.". A coil 3 of which a cross section is shown is provided at a predetermined position in gap 4 in the magnetic circuit, and produces the thrust in directions of arrows A and B by supplying electric current thereto in the forward direction or the reverse direction vertical relative to the drawing surface. In the area in the vicinity of the gap 4, there is leakage of the magnetic flux 5 which does not effect the driving operation of the coil 3. To achieve highly efficient driving operation in such electromagnetic driving circuits mentioned above, it is necessary to converge the magnetic flux in the area of the gap 4, especially in the area of the efficient portion of the coil 3. For example, one characteristic of the magnetic flux is that it tends to converge a highly permeable magnetic member. So, if a length of the gap 4 between the permanent magnet 1 and the yoke 2 is reduced, the magnetic flux density of the area of the gap 4 is increased compared with the area in the vicinity of the gap 4. However, when the length of the gap 4 is reduced by more than a predetermined length, it causes the cross section of coil 3 to be compressed. Namely, the thrust thereof becomes weakened because the effective length of the coil 3 decreases, or electric consumption is increased because of the increase of the resistance of the coil 3. Hence, reducing the length of the gap 4 is not preferable in view of improving the electromagnetic driving circuit.
In addition to that, to control an inclination of an optical axis of the objective lens which is driven at a high acceleration speed, the objective lens holder is elastically and movably supported in the focusing direction and the tracking direction on the carriage. In this elastic supporting mechanism, a damping member made of rubber is generally provided to supply damping resistance. However, in these elements, there have been problems such as imbalance of the resistance and deterioration of damping efficiency caused by temperature (temperature dependency). These cause the instability of the driving operation of the objective lens holder.
Apparatuses have been disclosed in U.S. Pat. No. 5,455,811 and U.S. Pat. No. 5,526,340 (disclosed by the same assignee of the present application) secure a high dynamic range, in the supporting mechanism for the objective lens which can be easily moved by a small driving force (low compliance), wherein the objective lens holder moving in a downward direction by the weight thereof can be prevented without supplying electric current to float the objective lens holder when the objective lens is not in a driving operation. These apparatuses are characterized by using an MID process (i.e., Molded Interconnection Device: which directly forms a printed circuit on a plastic injection molded product) to directly print an electric supplying circuit on a component which is made of the plastic injection molded product, with a magnetic force of the focusing and/or tracking permanent magnetos of the lens holder which is the component made by injection molding, to contain a magnetic material into an electric supplying printed circuit in a position where floating force (lifting force) is supplied to the objective lens holder. In this construction, when the objective lens holder is not in the driving operation, the weight of the objective lens holder can be at least partially and magnetically supported by the magnetic force of the permanent magnets on the carriage without supplying the electric current thereto, so that by supplying the magnetic force to the objective lens holder effectively to obtain the floating force in the area where the fluctuation of magnetic floating force is not easily effected, the magnetic circuit and the movable portion can be made small and light while maintaining the advantages in the above apparatuses.
In a positioning sensor which detects a position of the objective lens (especially in the tracking direction), since a photo sensor is generally utilized to detect a deviation of the objective lens from its neutral position based on light reflected by a reflecting surface which is provided on the objective lens holder, instability of the driving form of the object lens holder and the deviation from its neutral position hinders the accuracy in detecting the position of the objective lens.